Acute myelogenous leukemia (AML) is characterized by chromosomal translocations or point mutations that involve transcription factors, such as PML/RARa, AML1/ETO and C/EBPa. However, these gene arrangements and mutations are not sufficient to cause leukemia, and the nature of the additional required mutations has been elusive. It has recently been discovered that the FLT3 receptor tyrosine kinase is constitutively activated in 20-25% of AML as a consequence internal tandem repeat mutations (ITD). Furthermore, the FLT-ITD may occur simultaneously with each of the transcription factor mutations above. We hypothesize that the FLT3-ITD provides proliferative and/or survival signals to hematopoietic progenitors, and causes the acute leukemia phenotype through cooperation with transcription factor mutations that impair hematopoietic differentiation. We further hypothesize that FLT3-ITD leukemias can be treated with FLT3-specific small molecule inhibitors. In Specific Aim 1, we will analyze the transforming properties of FLT3-ITD in murine bone marrow transplantation assays. Specific Aims 2-4 will test the cooperative effect of FLT3-ITD with PML/RARa, AML1/ETO, or mutant C/EBPa, respectively, in murine models of leukemia. In Specific Aim 5, a FLT3-ITD "knock-in" mouse will be generated, in part to facilitate analysis of the structural requirements of cooperating transcription factor mutants. In each of these contexts, we will assess the therapeutic efficacy of a FLT3-specific small molecule inhibitor. Collectively, these experiments will determine the pathophysiologic significance of constitutive activation of FLT3 in human AML, and validate novel therapeutic approaches that target the FLT3-ITD.